Various press formed members press formed by using a blank (a raw material) worked by a thin plate such as a steel plate or an aluminum plate are used in a transport vehicle such as a motor vehicle, a vehicle, or a marine vessel, a structure, a machine, an electric appliance and the like. Particularly, in recent year, in the transport vehicle such as the motor vehicle, the vehicle, or the marine vessel, in order to cope with demand to a global environment protection based on a weight saving of a vehicle body, a high strength steel plate and an aluminum alloy plate have been positively employed as a blank material.
As the press formed member, for example, in a seat member, a chassis member and the like for a motor vehicle represented by a lower arm, there is frequently used a formed member provided with a flat base body 41 having a concave outer peripheral edge 42 in which a part of the outer peripheral edge is concaved, and a flange 43 formed by bending on an outer peripheral edge including the concave outer peripheral edge 42, as shown in FIG. 1. The flange 43 has a concave flange portion 44 formed by bending on the concave outer peripheral edge 42. FIG. 2 shows a formed member with a flange for explaining a general mode of the concave flange portion 44. The formed member has a base body 1 having a hourglass shape in a plan view, and a concave outer peripheral edge 2 thereof has an arch shape constructed by a convex curved portion 3 formed by a circular arc in which a center portion is convex to an inner side, and linear adjacent line portions 5 arranged on both sides thereof. A convex flange portion 4 is formed by bending in the convex curved portion 3 of the concave outer peripheral edge 2, an adjacent flange portion 6 is formed by bending in the adjacent line portions 5 on both sides thereof, and a concave flange portion 7 is formed by these flange portions 4 and 6. In this case, in the formed member in FIG. 2, the flange of the formed member is constructed by two opposing concave flange portions 7.
The formed member with the flange is manufactured by using a blank Bp having approximately the same shape as a shape obtained by expanding the formed member in a planar shape as the raw material and bending the flanges 7 and 43 in the base bodies 1 and 41. Specifically, the blank Bp is mounted to an upper flat portion of a die 31 and a pad 32, and the die 31 and the pad 32 are moved upward together, as shown in FIG. 3, and the blank Bp is held to a lower flat portion of a punch 33 by the pad 32 so as to prevent from being deviated its position, the die 31 is moved upward, and the flange is formed by bending on an outer peripheral edge of the base body in accordance with a cooperation between the punch 33 and the die 31, as shown in FIG. 4. In this case, in the illustrated example, the punch 33 is set to a fixed side, and the die 31 is set to a movable side, however, the die 31 may be set to the fixed side and the punch 33 may be set to the movable side, inversely.
If the formed member with flange is press formed by using the blank, there is generated such a deformation that the convex flange portion 4 in the center portion of the concave flange portion 7 is stretched in a peripheral direction (referred to as a “lateral direction”) of the concave outer peripheral edge 2 of the base body 1 (the deformation mentioned above is referred to as a “stretch-flange deformation”, and the forming generating the stretch-flange deformation is referred to as a “stretch-flange forming”). In a significant case, a crack (referred to as a “stretch-flange crack”) CR is generated in a lower end of the center portion of the convex flange portion 4, as shown in FIG. 2. Since a press forming performance is lowered generally in accordance with that a strength of the steel plate becomes higher, it is necessary to develop a steel plate and a forming technique which are excellent in formability, for press forming the formed member with flange having the concave flange portion by using the blank of the high strength steel plate. Further, since the stretch-flange crack comes to a problem, a shape such as a flange height is going to be limited.
Further, since the formability is interior to the steel plate, in the blank of the aluminum alloy plate, a crack tends to be generated in the flange end portion in which the stretch-flange deformation is generated, in the same manner as the case of using the high strength steel plate, in a large-sized and complicated panel formed product, for example, a door outer plate, a seat pan and an oil pan mechanism, in recent years.
Accordingly, in the press forming accompanying the stretch-flange deformation, various crack preventing techniques have been discussed. For example, in Japanese Unexamined Patent Publication No. 2004-130350 (Patent Document 1), there is disclosed a technique preventing a crack by applying an electromagnetic forming to the forming of the shape portion in which the stretch-flange deformation is generated, and generating an extremely high speed stretch-flange deformation, in the press formed product of the aluminum alloy motor vehicle panel. Further, in Japanese Unexamined Patent Publication No. 2002-113527 (Patent Document 2), there is disclosed a technique preventing the stretch-flange crack of the high strength steel plate from being generated by setting a steel plate temperature during forming to 400° C. or higher and 1000° C. or lower at a time of forming the stretch flange, and generating a dynamic recovery of dislocations during work so as to make the dislocations hard to be piled up.
However, in the forming method described in Patent Document 1, since the step of the electromagnetic forming is increased in addition to the normal press forming, it is disadvantageous in the light of productivity, and the high strength steel plate comes short of being worked in comparison with the aluminum alloy in the light of a working force of the electromagnetic forming, so that this forming method is hard to be applied to the forming of the high strength steel plate. Further, in the forming method of Patent Document 2, an increase of the step number is suppressed by finding the minimum places heating the steel plate, however, it is unavoidable that a lead time to an end of the forming is increased in comparison with a conventional cold press, and this forming method is disadvantageous as an industrial forming method in the light of an equipment cost and a productivity.